Neuronal axon degeneration is a hallmark of all neurodegenerative disorders including Alzheimer's, Luo Gehrig's, and Huntington's disease, as well as neural injury (e.g. stroke, traumatic brain injury and spinal cord injury). Moreover, loss f axons and synapses via degeneration is thought to be a main cause of cognitive decline, movement disorders, and paralysis associated with these pathologies. Despite their importance as therapeutic targets, very little is known about the mechanisms underlying axon loss. Beyond pathological situations, axon degeneration is also an important aspect of nervous system refinement during development. One particular aspect of developmental degeneration, that has yet to be discovered in injury-induced degeneration, is the ability to respond to axon derived extrinsic factors. In this proposal we examine whether extrinsic initiators of developmental axon degeneration programs are also used to coordinate injury-induced axon degeneration (also known as Wallerian degeneration) and regeneration. In particular, we examine family members of tumor necrosis factor receptor (TNFR), which are known to be responsible for developmental neuron death and axon degeneration. We present preliminary evidence that these receptors do indeed coordinate injury-induced axon degeneration in tissue culture as well as in mice. These are the first receptors discovered, that when lost, prevent Wallerian degeneration. In Aim1 we develop an experimental paradigm to rapidly and reproducibly assess axon degeneration in neurons from neonatal and adult mice. This tool will be useful as we continue to explore whether TNFR family members and associated signaling pathways are capable of coordinating Wallerian degeneration in sympathetic and sensory neurons grown in tissue culture (microfluidic devices). In Aim2, we examine whether extrinsic cues derived from injured axons are capable of expediting developmental axon degeneration. We then leverage this as a bioassay to biochemically characterize these extrinsic factors. Finally, in Aim3 we examine the influence of these pathways on axon degeneration and regeneration in a sciatic nerve axotomy model using mice lacking one or more TNFR family members, their ligands, and/or components of associated pathways. Identifying extrinsic factors that influence Wallerian degeneration has broad implications, not only for understanding how axon degeneration occurs, but also for the range of therapeutic approaches that can be employed to treat pathology-associated degeneration. Our expertise in mouse genetics, developmental neuroscience, cytokine signaling, and biochemical analysis places us in a unique position to delineate mechanisms that have eluded the field for decades.